By all welding a positive and a negative cable are used where the negative cable is the ground cable which is fastened to the construction to be welded. The welding is preformed by the positive cable to which an electrode holder or a feeding piston is mounted. To fasten the ground cable to the construction a grounding terminal is used.
Known grounding terminals comprise among other things earthing clamps, that grip over the construction, example given a pipe, by using a mechanical force providing electrical contact with the construction. However, earthing clamps can not be Fastened to all geometrical surfaces as they are dependent upon being able to grip around the object. The electrical connection is dependent upon the jaws of the earthing clamps gripping around the construction to give sufficient contact. The mechanical force is often provided by screws or mechanical springs. that after a while can get loose and thereby give poorer contact with the construction. If an earthing clamp is to be fastened on bigger constructions, an ear must first be welded to the construction to which the earthing clamp then can be fastened.
Other known grounding terminals are based on magnetism, and comprise as terminals with permanent magnets and terminals based on the use of electromagnets. Such grounding terminals can be used when the construction to be welded is made of a ferromagnetic material. For example, there are described in the patents U.S. Pat. No. 2,287,112 and U.S. Pat. No. 2,437,833 magnetic grounding terminals based on an electromagnet. When the current is switched on, the terminals are magnetized and the contact surface is drawn towards the workpiece. Electromagnetic grounding terminals must however have their own power supply and their own conductor from the power supply to the coil in the electromagnet of the terminal. Grounding terminals with permanent magnets are disclosed in U.S. Pat. No. 2,429,526 and U.S. Pat. No. 4,676,562. The grounding terminal in
U.S. Pat. No. 4,676,562 is designed to be held and fastened by one hand. The terminal will however be hard to unfasten from the workpiece. James Mortem.TM. has also put on the market a magnetic grounding terminal based on permanent magnets. Known grounding terminals with permanent magnets have generally the disadvantage that the magnetic force can not be"turned off" in the way the current in an electromagnet can be turned off, or otherwise be moved away from the contact surface with the construction. These magnetic grounding terminals will in a typical industrial environment also attract metal shavings that fasten on the contact surface. This is highly unwanted as it will give poorer contact between the workplace and the grounding terminal.
To obtain a good ground connection between the grounding terminal and the workpiece to be welded, the contact surface on the construction must also be free of paint, rust and oxide scale. This is now done by using a separate scraper in some form. The welder must therefor be equipped with several tools during welding.